Mastroianni was recalled from Triple-A Rochester when third baseman Danny Valencia was optioned to Rochester after Wednesday's game.
It marked the 26-year-old's second career start, as he made his Major League debut with the Blue Jays, coincidentally, on Aug. 24, when he went 0-for-2. Mastroianni was claimed off waivers by the Twins from the Blue Jays on Feb. 9.
"This is like a dream come true to me," Mastroianni said. "Obviously coming over to the Twins this offseason was a big deal, changing teams for the first time. Everyone's been real nice and really supportive and helpful from Day 1. I'm just really excited to be here. To get that call last night was a big deal. It's a big moment in my career."
Mastroianni began the season at Double-A New Britain, batting .143 with one double in nine games, before being promoted to Rochester on April 14. In 19 games with the Red Wings, he hit .346 with two doubles, two triples and 10 stolen bases.
"He's been playing well down there and running all over the place, and I want to see him out there, see what happens," Twins manager Ron Gardenhire said. "He's been getting it done. That's why he's up here. We talked about the guys who were playing the best."